Abstract
BackgroundMating swarm segregation in closely related insect species may contribute to reproductive isolation. Visual markers are used for swarm formation; however, it is unknown whether they play a key role in swarm location, species segregation and sex aggregation.MethodsUsing two sympatric closely related species of the Anopheles gambiae complex, An. coluzzii and An. gambiae (s.s.), we investigated in both laboratory and semi-field conditions (i) whether males of the two species use visual markers (black cloths) to locate their swarm; and (ii) whether the presence/absence and size of the marker may differentially affect swarm characteristics. We also investigated whether conspecific virgin females use these markers to join male swarm sites.ResultsWe showed that males of the two species used visual markers but in different ways: An. coluzzii swarm right above the marker whereas An. gambiae (s.s.) locate their swarm at a constant distance of 76.4 ± 0.6 cm from a 20 × 20 cm marker in the laboratory setup and at 206 ± 6 cm from a 60 × 60 cm marker in the semi-field setup. Although increased marker size recruited more mosquitoes and consequently increased the swarm size in the two species, An. coluzzii swarms flew higher and were stretched both vertically and horizontally, while An. gambiae (s.s.) swarms were only stretched horizontally. Virgin females displayed a swarm-like behavior with similar characteristics to their conspecific males.ConclusionsOur results provided experimental evidence that both An. coluzzii and An. gambiae (s.s.) males use ground visual markers to form and locate their swarm at species-specific locations. Moreover, the marker size differentially affected swarm characteristics in the two species. Our results also showed that virgin females displayed a swarm-like behavior. However, these “swarms” could be due to the absence of males in our experimental conditions. Nevertheless, the fact that females displayed these “swarms” with the same characteristics as their respective males provided evidence that visual markers are used by the two sexes to join mating spots. Altogether, this suggests that visual markers and the way species and sexes use them could be key cues in species segregation, swarm location and recognition.
Highlights
Mating swarm segregation in closely related insect species may contribute to reproductive isolation
While An. coluzzii female swarms were very similar to male swarms, those formed by An. gambiae females were less stable with one or a few females coming at the swarming location, doing a few loops and leaving
Our results provide the experimental evidence that the virgin females of both An. gambiae (s.s.) and An. coluzzii use ground markers to join conspecific male swarm sites
Summary
Mating swarm segregation in closely related insect species may contribute to reproductive isolation. This behavior can be a response to environmental heterogeneities or needs for social interactions [1,2,3]. They occur around an attractive resource and the individuals might disperse if this resource wanes or once the environmental heterogeneity is removed [3]. In some cases, gathering may contribute to the reproductive isolation [3, 4] of species with closely related species distinguishing themselves from each other thanks to species-specific aggregation preferences [4,5,6,7]. Species segregation can occur either in space through the preference for different aggregation sites or over time with aggregations occurring at a same location but at different times [3, 5, 8,9,10,11]
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